The invention relates to an anionically polymerized, impact-modified polystyrene with high stiffness and toughness, and also to a process for its preparation.
There is a variety of continuous and batch processes in solution or suspension for preparing impact-modified polystyrene, As described in Ullmanns Enzyklopxc3xa4die, Vol A21, VCH Verlausesellschaft Weinheim 1992, pp. 615-625. In these processes a rubber, usually polybutadiene, is dissolved in monomeric styrene and the styrene is free-radical polymerized via thermal or peroxidic initiation. Besides homopolymerization of styrene, graft polymerization of styrene onto polybutadiene also takes place. Consumption of the monomeric styrene with the formation of polystyrene gives rise to a xe2x80x9cphase inversionxe2x80x9d. The properties of the impact-modified polystyrene are determined by the morphology, the particle size and the particle size distribution of the disperse rubber particles. These are dependent on a variety of process parameters, such as the viscosity of the rubber solution and sheer forces during stirring.
Processes for preparing thermoplastic molding compositions via anionic polymerization of styrene in the presence of styrene-butadiene block copolymers are disclosed, for example, in DE-A-42 35 978, WO 96/18666 or U.S. Pat. No. 4,153,647. The resultant impact-modified products Slave lower residual monomer and oligomer contents than products obtained by free-radical polymerization. However, anionically polymerized products generally have inadequate toughness.
WO 98/07766 describes the Continuous preparation of impact-modified molding compositions using styrene-butadiene rubbers. The rubbers were polymerized anionically using retardant additives, such as alkyl compounds of alkaline-earth metals, of zinc or of aluminum in styrene as solvent. However, their butadiene blocks always-contain small amounts of copolymerized styrene.
Since the reaction mechanisms for free-radical and anionic polymerization of styrene differ, the process parameters known for free-radical preparation of impact-modified polystyrene are not directly transferable to the anionic polymerization of styrene in the presence of rubbers. For example, exclusive use of homopolybutadiene is not possible since no graft reactions occur in the anionic polymerization of styrene.
It is an object of the present invention to provide an anionically polymerized, impact-modified polystyrene with high stiffness and toughness. The impact-modified polystyrene should be low in residual monomers and residual oligomers and have a low ethyl benzene content. A further object was to find an anionically polymerized impact-modified polystyrene with cell-particle morphology, and also a process for its preparation.
We have found that this object is achieved by obtaining a novel impact-modified polystyrene via anionic polymerization of styrene in the presence of a rubber, the rubber used being a styrene-butadiene-styrene three-block copolymer with a styrene content of from 5 to 75% by weight, preferably from 25 to 50% by weight.
A specimen produced to ISO 3167 has a yield stress of at least 24 MPa, measured at 23xc2x0 C. to DIN 53455 and a hole notched impact strength of at least 11 kJ/m2, measured at 23xc2x0 C. to DIN 53753.
The specimens produced to ISO 3167 by injection molding preferably have a yield stress of at least 27 MPa, particularly preferably from 30 to 50 MPa, measured at 23xc2x0 C. to DIN 53455 and a hole notched impact strength of at least 13 kJ/m2, particularly preferably from 15 to 30 kJ/m2, measured at 23xc2x0 C. to DIN 53753. The values for yield stress are generally from 20 to 30% lower for compression-molded specimens (e.g. DIN 16770, Part 1) than the values for injection-molded specimens, and the hole notched impact strength values are generally from 30 to 40% lower.
The ratio of yield stress to hole notched impact strength generally has a numerical value of at least 1.5, preferably at least 2.
The disperse soft phase of the novel impact-modified polystyrene preferably comprises a styrene-butadiene block copolymer and has cell-particle morphology.
The novel impact-modified polystyrene may be obtained by anionic polymerization of styrene in the presence of a rubber. The rubber used here comprises a styrene-butadiene-styrene three-block copolymer with a styrene content of from 5 to 75% by weight, preferably from 25 to 50% by weight or a styrene-butadiene two-block copolymer or a mixture of a styrene-butadiene two-block copolymer with a homopolybutadiene, where the styrene-butadiene two-block copolymer or, respectively, the mixture has a styrene content of from 25 to 75% by weight, preferably from 30 to 50% by eight.
It is particularly preferable for the rubber used to comprise an asymmetric styrene-butadiene-styrene three-block copolymer S1-B-S2, where S1 is a styrene block with a weight-average molar ass Mw of from 5000 to 100000 g/mol, preferably from 10000 to 40000 g/mol, B is a butadiene block with a weight-average molar mass Mw of from 12000 to 500000 g/mol, preferably from 70000 to 250000 g/mol and S2 is a styrene block with a weight-average molar mass Mw of from 30000 to 300000 g/mol, preferably from 50000 to 200000 g/mol.
The residual butadiene content of the homopolybutadiene and the styrene-butadiene block copolymer used should be less than 200 ppm, preferably less than 50 ppm, in particular less than 5 ppm.
The rubber content, based on the impact-modified polystyrene, is usefully from 5 to 25% by weight.
The conversion, based on styrene in the hard matrix, is generally above 90%, preferably above 99%. The process may in principle also give complete conversion.
In place of styrene it is also possible to use other vinylaromatic monomers for the polymerization of the hard matrix or of the styrene blocks in the block copolymers. Examples of other suitable monomers are xcex1-methylstyrene, p-methylstyrene, ethylstyrene, tert-butylstyrene, vinyltoluene, 1,2-diphenylethylene and 1,1-diphenylethylene, and mixtures. Particular preference is given to the use of styrene.
The rubbers may also comprise other dienes instead of butadiene, for example 1,3-pentadiene, 2,3-dimethylbutadiene, isoprene or mixtures of these.
The anionic polymerization initiators usually used are mono-, bi- or multifunctional alkyl, aryl or aralkyl compounds of alkali metals. Useful compounds are organolithium compounds, such as ethyl-, propyl-, isopropyl-, n-butyl-, sec-butyl-, tert-butyl-, phenyl-, diphenyl-, hexyl-, hexamethylenedi-, butadienyl-, isoprenyl- and polystyryllithium, or the multifunctioiial compounds 1,4-dilithiobutane, 1,4-dilithio-2-butene or 1,4-dilithiobenzene. The required amount of alkali metal organyl compound depends on the molecular weight desired and on the nature and amount of the other metal organyl compounds used, and also on the polymerization temperature. It is generally from 0.002 to 5 mol %, based on the total amount of monomers.
The polymerization may be carried out with or without a solvent. The polymerization usefully takes place in an aliphatic, isocyclic or aromatic hydrocarbon or hydrocarbon mixture, such as benzene, toluene, ethylbenzene, xylene, cumene, hexane, heptane, octane or cyclohexane. Preference is given to solvents with a boiling point above 95xc2x0 C., particularly toluene.
Additives which reduce the polymerization rate, known as retarders, as described in WO 98/07766, may be added in order to control the reaction rate. Examples of suitable retarders are metal organyl compounds of an element of the second or third main group of the periodic table or the second transition group. Examples of organyl compounds which may be used are those of the elements Be, Mg, Ca, Sr, Ba, B, Al, Ga, In, Tl, Zn, Cd, Hg. Preference is given to the use of the magnesium and aluminum organyl compounds. For the purposes of the present invention, organyl compounds are the organometallic compounds of the elements mentioned having at least one metal-carbon a bond, in particular the alkyl or aryl compounds. The metal organyl compounds may also contain, on the metal, hydrogen, halogen or organic radicals bonded via heteroatoms, such as alcoholates or phenolates. The latter are obtainable, for example, by partial or complete hydrolysis, alcoholysis or aminolysis. It is also possible to use mixtures of different metal organyl compounds.
Suitable magnesium organyl compounds are those of the formula R2Mg, where the radicals R, independently of one another, are hydrogen, halogen, C1-C20-alkyl or C6-C20-aryl. Preference is given to dialkylmagnesium compounds, in particular the ethyl, propyl, butyl, hexyl or octyl compounds which are available commercially. Particular preference is given to the use of (n-butyl) sec-butyl magnesium, which is soluble in hydrocarbons.
Aluminum organyl compounds which may be used are those of the formula R3Al, where the radicals R, independently of one another, are hydrogen, halogen, C1-C20-alkyl or C6-C20-aryl. Preferred aluminum organyl compounds are the trialkylaluminum compounds, such as triethylaluminum, triisobutylaluminum, tri-n-butylaluminum, triisopropylaluminum and tri-n-hexylaluminum. Particular preference is given to the use of triisobutylaluminum. Other aluminum organyl compounds which may be used are those produced by partial or complete hydrolysis, alcoholysis, aminolysis or oxidation of alkylaluminum or aryl aluminum compounds. Examples of these are diethylaluminum ethoxide, diisobutylaluminum ethoxide, diisobutyl(2,6-di-tert-butyl-4-methylphenoxy)aluminum (CAS No. 56252-56-3), methylaluminoxane, isobutylated methylaluminoxane, isobutylaluminoxane, tetraisobutyldialuminoxane and bis(diisobutyl)aluminum oxide.
The polymerization of the styrene is particularly preferably carried out in the presence of a trialkylaluminum and/or dialkylmagnesium compound.
The retarders described do not generally act as polymerization initiators. However, it has been found that the polymerization of the hard matrix can be carried out without any other addition of an anionic polymerization initiator if direct use is made of a rubber solution which has been initiated via anionic polymerization using an anionic polymerization initiator and then terminated by chain-termination and/or by coupling. In this case the metal alkyl compounds, which otherwise only act as retarders, can initiate the polymerization of the hard matrix. This makes metering and control simpler than when an initiator/retarder mixture is used.
A magnesium content of from 0.1 to 100 mmol/kg and/or an aluminum content of from 0.01 to 50 mmol/kg, based in each case on the impact-modified polystyrene, does not generally significantly impair mechanical properties.
To increase elongation at break, from 0.1 to 10% by weight, preferably from 0.5 to 5% by weight, of mineral oil, based on the impact-modified polystyrene, may be added in the novel process.
The polymerization of the hard styrene matrix may be carried out batchwise or continuously in stirred-tank reactors, circulating reactors, tubular reactors, tower reactors or rotating disk reactors, as described in WO 97/07766.
The content of styrene monomers in the impact-modified polystyrene is generally not more than 50 ppm, preferably not more than 10 ppm, and the content of styrene dimers and of styrene trimers is not more than 500 ppm, preferably not more than 200 ppm, particularly preferably less than 100 ppm. The ethylbenzene content is preferably below 5 ppm.
It can be useful to achieve crosslinking of the rubber particles by using an appropriate temperature profile in a devolatolizer or vented extruder and/or by adding peroxides, in particular those with a high decomposition temperature, for example dicumyl peroxide.
Other conventional auxiliaries, such as stabilizers, lubricants, flame retardents, antistats, may be added to the novel polymers.
The novel impact-modified polystyrene is suitable for producing fibers, films or moldings.